This invention relates to semiconductor devices and, more particularly, to buried heterostructure lasers.
Semiconductor diode lasers fabricated from the InGaAsP/InP materials system are currently of interest for application in optical communication systems operating at 1.0-1.6 .mu.m. Of the wide variety of possible laser structures which can be fabricated, those which utilize a real refractive index waveguide, such as a buried heterostructure (BH), have recently been shown to have advantages with respect to the absence of both self-pulsations in their outputs and kinks in their light-current behavior. [R. J. Nelson et al, Applied Physics Letters, 37, 769 (1980).] For most refractive-index-guided laser structures, the fabrication sequence usually requires an etching step to form a mesa which ultimately defines the lateral dimensions of the optical cavity and the active region of the laser. Since the transverse mode characteristics of this cavity depend on, among other factors, the cavity geometry and dimensions, it is important to exercise a high degree of control over these parameters during the etching process if high yields of single transverse mode devices are to be obtained. For example, typically the dimensions of the active region of a BH laser, which operates in the fundamental transverse mode, are 0.15 .mu.m in thickness and a maximum of 2.0 .mu.m in width. Associated with the problem of strict geometrical and dimensional control is the problem of mask undercutting. This undercutting is often unpredictable, leading to a loss of dimensional and geometrical control over the desired mesas. In the case of a BH laser, having a 2.0 .mu.m wide active layer, undercutting of only 1.0 .mu.m on each side of the mesa results in complete loss of the mesas on the wafer. Even in cases where the mask undercutting is predictable and can be allowed for, a large amount of mask overhang can lead to problems in later processing steps, such as LPE regrowth where local growth dynamics can be adversely affected. It is clear, therefore, that the etchant system and etching technique used to fabricate mesas for these laser structures must allow for precise geometrical and dimensional control with little or no undercutting. In addition, the etchant of choice should leave a contaminant-free surface which is smooth and free from pits or other defects which may lead to problems in subsequent processing steps.
Bromine-methanol is one etchant which has been utilized successfully as a preferential etchant in the fabrication of InGaAsP/InP BH lasers [M. Hirao et al, Journal of Applied Physics, 51, 4539 (1980); R. J. Nelson et al, IEEE Journal of Quantum Electronics, 17, 202 (1981)] and buriedo-waveguide-heterostructure lasers [R. B. Wilson et al, International Electron Devices Meeting, Technical Digest, 370 (1980)]. In those reports, however, emphasis was placed on device results rather than on the mesa etching process and the mask undercutting problem.